As youroo pointed out, in most rigs winterizing the BTF takes just a few seconds. Just use your outside shower connection and shoot a bit of pink through it. Is it 'necessary' IMHO debatable, but since it is usually easy to do, why not. I do mine every year on my first winterization. On every winterization after that I don't since we don't use the flusher when winter camping. To each their own though.
This aside, I will be checking mine in the spring and I think I'll probably swap it out for the one linked to in this thread.
2015 F-350 SRW 6.7 PSD
2014 Crusader 295RST
1 Fantastic DW
1 Amazing DD
1 Beagle that will be missed (RIP Zeus)